1. Field of the Invention
The present invention relates to a plastic magnetic material for insert molding to be used for motors and the like, particularly to a polyamide plastic magnetic material excellent in fluidity, and further to a plastic magnet made of the same, highly flexible with no deterioration in strength, and free from cracking due to shrinkage or thermal shock during a fabrication process.
2. Description of the Related Art
A plastic magnetic material can be used in insert molding, in which a motor shaft, for example, can inserted in a molding die. The insert molding is excellent in productivity and thus can reduce manufacturing cost of magnets. Also, the magnets made by the insert molding are excellent in dimensional precision, which expands the usage thereof. Meanwhile, an increasing amount of magnetic powder is used in a plastic magnetic material for a higher performance, and it is strongly demanded that a plastic magnetic material be more fluid and that a magnet made thereof be stronger and more flexible. A plastic magnet made of anisotropic magnet powder is produced in such a way that a molding process is conducted in a magnetic field so as to direct an axis of easy magnetization as desired in order to obtain a high performance. In this process, if the fluidity deteriorates, a magnet performance as expected cannot be realized.
The fluidity deteriorates because of melt viscosity of composition increasing due to increased content of magnetic powder in the plastic magnetic material. To overcome the problem, it has been proposed that lubricant such as metal soap be added to the plastic magnetic material.
The increased content of magnetic powder in the plastic magnetic material causes also deterioration in strength. Since the plastic magnet material has an expansion coefficient different from that of a metal to be insert molded, a plastic magnet suffering deterioration in strength is more liable to crack due to shrinkage at cooling or thermal shock during the molding process. The plastic magnet is required to be flexible so as to absorb difference in expansion coefficient between metallic magnetic powder and plastic resin but its flexibility deteriorates also due to the increased content of magnetic powder. To overcome the problem of deterioration in strength and flexibility of the plastic magnet, it is proposed to use a copolymer comprising polyamide and rubber as a plastic resin (Japanese Patent Laid-open No. Sho 57-187910), or to use polyamide elastomer containing a polyether group together with polyamide (Japanese Patent Laid-open No. Sho 63-122106).
FIG. 1 is a load-displacement curve showing one example of the results of a three-point bending test conducted on plastic magnets made respectively of (a) a plastic magnetic material of nylon 12-strontium ferrite, (b) a plastic magnetic material with addition of metal soap for better fluidity, and (c) a plastic magnetic material with addition of polyamide elastomer for providing flexibility. The three-point bending test is conducted in such a way that a plate-like sample is supported at two points on its bottom face and is destroyed with a load applied to its top face at the center between the two points. A load value measured at the breaking point is a breaking strength, where a larger breaking strength means a larger strength of the sample. And a displacement at the breaking point is a measure of flexibility, where a larger displacement means a larger flexibility of the sample.
As apparent from FIG. 1, addition of metal soap, while contributing toward increasing the fluidity of the plastic magnetic material, causes deterioration in the strength of a magnet to be made of the material. Therefore, the metal soap as a lubricant must not be added in a large amount.
Use of polyamide elastomer increases the flexibility of a magnet to be made of the material, but the polyamide elastomer has a lower flexural strength characteristic than nylon 12 and therefore reduces the strength of the plastic magnet. The use of polyamide elastomer also invites deterioration in the fluidity of the plastic magnetic material.
As a result, lubricant, while making the plastic magnetic material more fluid, invites deterioration in the strength of the plastic magnet made of the material, and polyamide elastomer, while making the plastic magnet more flexible, also invites deterioration in the strength of the magnet.
Accordingly, it is an object of the present invention to provide a plastic magnetic material which has an increased fluidity and improves flexibility of a plastic magnet to be made of the plastic magnetic material without deteriorating the strength of the plastic magnet, and which does not crack due to shrinkage and thermal shock during the manufacturing process.
In order to achieve the above object, the present inventors conducted researches and found that a polyamide plastic magnetic material consisting of magnetic powder and polyamide resin, when mixed with bis unsaturated fatty acid amide, improves the fluidity of the plastic magnetic material, and that the bisamide reacts on the polyamide resin thereby enhancing the strength and flexibility of plastic magnets made of the plastic magnetic material.
It is disclosed in Japanese Patent Laid-open No. Sho 58-158903 that organometallic compound containing an amino group and a bisamide are added to a plastic magnetic material consisting magnetic powder and nylon, thereby lowering the melting viscosity of the plastic magnetic material to improve moldability. However, the Patent published does not disclose at all that a plastic magnet made of a plastic magnetic material produced by adding a bis unsaturated fatty acid amide is excellent in flexibility, which is found for the first time by the present invention.
According to a first aspect of the present invention, there is provided a polyamide plastic magnetic material comprising: magnetic powder; a polyamide resin; and a bis unsaturated fatty acid amide which is represented by the formula below and present in an amount equal to 3 to 30 mass % of the polyamide resin:
R1xe2x80x94CONHxe2x80x94R3xe2x80x94NHCOxe2x80x94R2 
where R1 and R2 are an unsaturated hydrocarbon group having at least one double bond and R3 is a hydrocarbon group, and the three components are kneaded to finalize the polyamide plastic magnetic material.
According to a second aspect of the present invention, the polyamide plastic magnetic material includes magnetic powder formed of strontium ferrite.
According to a third aspect of the present invention, the polyamide plastic magnetic material includes magnetic powder formed of barium ferrite.
According to a fourth aspect of the present invention, there is provided a polyamide plastic magnet, which is fabricated by a molding process and made of the polyamide plastic magnetic material according to any one of the first to third aspects.
According to a fifth aspect of the present invention, the molding process is insert molding.